An extended Higgs sector may allow for new scalar particles well below the weak scale. In this work, we present a detailed study of a light scalar S with enhanced coupling to leptons, which could be responsible for the existing discrepancy between experimental and theoretical determinations of the muon anomalous magnetic moment. We present an ultraviolet completion of this model in terms of the lepton-specific two-Higgs-doublet model and an additional scalar singlet. We then analyze a plethora of experimental constraints on the universal low energy model, and this UV completion, along with the sensitivity reach at future experiments. The most relevant constraints originate from muon and kaon decays, electron beam dump experiments, electroweak precision observables, rare Bd and Bs decays and Higgs branching fractions. The properties of the leptonic Higgs portal imply an enhanced coupling to heavy leptons, and we identify the most promising search mode for the high-luminosity electron-positron colliders as e++e-→τ++τ-+S→τ++τ-+ℓ+ℓ, where ℓ=e, μ. Future analyses of existing data from BABAR and Belle, and from the upcoming Belle II experiment, will enable tests of this model as a putative solution to the muon g-2 problem for mS<3.5 GeV.
Bibliographical noteFunding Information:
The work of B. B. is supported in part by the U.S. Department of Energy under Grant No. DE-SC0015634. The work of D. M. is supported by the U.S. Department of Energy under Grant No. DE-FG02-96ER40956.